The Portable Microphone Preamplifier circuit is primarily intended to provide the usual stereo home amplifier with microphone input. The power supply from the battery is a good compromise, in this way the input circuit is free of low frequency and the power connection to the amplifier is simpler, due to the absence of power cord and power supply. Using the Stereo Microphone, the circuit capability must be doubled. In this case, two separate levels of control are better than a dual-ganged stereo potentiometer. A low current (about 2mA) will ensure the battery is more durable.
Schematic of Portable Microphone Preamplifier Circuit:
Component:
P1 = 2K2 Linear Potentiometer
R1, R2, R3 = 100K 1/4 Watt
R4 = 8K2 1/4 Watt
R5 = 68 Ohm 1/4 Watt
R6 = 6K8 1/4 Watt
R7, R8 = 1K 1/4 Watt
R9 = 150 Ohm 1/4 Watt
C1 = 1uF / 63V Polyester Capacitor
C2, C3, C4 = 100uF / 25V Electrolytic Capacitors
C5 = 22uF / 25V Electrolytic Capacitor
Q1 = BC560C 45V 100mA PNP Transistor
Q2 = BC550C 45V 100mA NPN Transistor
J1 = Jack socket (Mono 3 or 6mm)
How it works :
This circuit is based on low noise noise, high gain two phase PNP fan NPN amplifier transistors, using DC negative feedback through R6 to stabilize proper working conditions. The output level is attenuated by P1 but, at the same time, the stage gain is lowered because of the increased value of R5. The unusual relationship of P1, helps in obtaining high input headroom, which allows to overcome various input sources (0.2 to 200mV RMS for 1V RMS output).
Note:
- The harmonic distortion is about 0.1% @ 1V RMS output (all frequencies).
- Maximum input voltage (maximum cursor level set maximum) = 25mV RMS
- Maximum input voltage (level of cursor control set in middle position) = 200mV RMS
- Covering the circuit in a metal casing is highly recommended.
- Simply connect the output of this device to the Aux input of your amplifier through the appropriate cables and connectors.